I noticed that these rifles are chambered in multiple .45 calibers. .45-70, .45-90, .45-110, and .45-120. Now my question is could you buy a shaprs rifle chambered in .45-120 and shoot .45-70 out of it? Reason being lower cost ammunition, and lower recoil.

You could do what you are talking about, but you would have side effects.  I shoot a Pedersoli Sharps in .45-70, throwing 530gr cast lead bullets over about 67gr of FFg black powder. I've read quite a bit about shooting these big rifles.  I'm talking black powder loads.  Anything in 'white powder' larger than .45-70 would likely be too painful to shoot.  I believe the difficulty would be that the bullet would be so far back from the throat, that you would experience some blowby, increased fouling and increased leading, and decreased accuracy.  

Most Sharps type rifles can be really picky about things like the cartridge length and the bullet seating depth in regards to the distance from the throat to the bullet.  (i.e., how far the bullet has to travel before it engages the rifling)   Some guys even seat the bullet IN the bore and insert a cartridge behind it.  

If you really need .45-120 oomph, but want to save $$, well, you're in a quandry.  -120 brass is expensive, but the good news is, you can use this straight walled brass many times.  

One compromise you might look at is getting a rifle chambered in .45-70, and shoot it a while, ESPECIALLY if you're learning to load BPCR (which is what these rifles are MEANT for), and then if you decide you want more power behind it... have it reamed out to -90, -100, -110 or -120.   (If you go for those longer cartridges, you better make sure your dental work is good... I hear they'll rattle your brains and your teeth out!)

My aforementioned loads out of my rifle (with a 34" target barrel, weighing in at almost 13lb) are very pleasant to shoot, and can be very accurate.  I've gotten some 5 and 10 shot groups to as small as about 1.1~1.25" at 100 yards.   I haven't shot it much this year, and pretty  much missed the silhouette season... maybe next year I'll get into it a little more.  

Thanks Matthew _Q, thats what I thought. Iwas hoping I would get lucky and be able to shoot both calibers withou having to buy two rifles. Guess not.

The Shiloh Forums is a great place to get lots of info.  I learned most of what I know there.  

It's still worth it if you only get one caliber.  It's very satisfying to lob that bigass chunk of lead just about as accurately as any new whiz-bang rifle with a $1000 scope on it.   And .45-70 is quite capable when it comes to BPCR.  It's more than enough for Silhouette and Mid Range matches, which go out to about 600 yards.   You can use it at 1000 for target with success, but getting out that far and further is where the longer cartridges with more oomph start to show a difference.

I've learned to load, cast bullets, mix my own lube, use a vernier tang sight... I always draw some attention at the range when I break out my big boomstick!  

Let me know if you have any questions.  

Thanks Matthew_Q, when I get ready to buy I will talk to you.